Superindelible Marker
I don't write with protagonists that young, so I'm not exactly sure
sweatdrop I suppose the reasonable thing to think is do they have the skills, experience, and mental ability to do what you want you want them to do? As in, I
personally don't think a standard 14 year old would be able to work that well in a mystery unless they were fodder, because I don't think they're mentally advanced enough to do the work. A 16 or 17 year old, though, would have a bit more experience under their belt and could probably do more. It really depends on what you want to do with the characters and who your audience is.
The main thing for antagonists is, for the love of all, make them believable. They have to believe their own cause is right - no human or creature wants to think they are the bad guy. The easiest thing I have found for making decent antagonists is give them a backstory of their own, just as detailed as the protagonist's. Then you'll know how they react to certain situations as well as why they are trying to thwart the protagonist. It'll also give the antagonist consistency and you'll be *less* likely to fall into overused tropes with dialogue/traps/scenes that have been written hundreds of times before, because they don't necessarily fit the way your antagonist is built. I suppose the short way of saying it is make your antagonist as three dimensional as your protagonist(s). No one likes a flat villain.
I have about two protagonist: Alex and Caleb, who both attend the same high school. Despite it being a mystery, there is no murder. It's all mainly blackmail and such. The main antagonist(s) (if you take into account he has a brother that's been hidden from him his whole life, who also becomes an antagonist) I feel is very believable. He's grown up in the shadow of his brother, Caleb, and want's to be bigger than just being his brother, yet it doesn't really start out that way. He starts out blackmailing to protect his brother, but instead realizes his status and in the middle, turns against the both of them out of pure jealousy. But when he catches wind of trouble brewing between Alex and Caleb (because they get off on the worst start ever) he decides to blackmail Alex to keep him quiet. Some things happen, and soon Alex and Caleb are forced to work together to get behind why Jackson (the brother of the protagonist, Caleb) is doing the things he's doing. I believe his motive is
very justified in a sense that his parents kind of ******** him over. So now, he's taking his revenge upon anyone who sort of gets in his way. But the main reason for the blackmail is that Alex learns a terrifying secret about Caleb, and Jackson does all this to shut him up.
God, I did not mean to rant or bore you with that little synopsis, but that's the jidst of it.